PIE laringeals

Hello,

What theories are there about the phonetic value of the PIE laryngeals? I've read somewhere they might have been pronounced as x', x and xw respectively (to fit into the pattern set by k', k and kw etc.). However, it doesn't seem plausible to me - if xw turned e into o, why didn't kw or gw? According to me, h1 could have been a glottal stop, since it didn't affect a flanking e (excluding the lengthening), and disappeared early, even in Anatolian. H2 on the other hand might have been a sort of pharyngeal. As for h3, I've got no idea. Are there any sounds which display similar effects on vowels in other languages of the world? And what about the reflexes of the laryngeals in Slavic and Baltic - they also yielded prosodic (tonal) features.

David